Advanced alloys, including nickel-based superalloys, intermetallics of titanium-aluminum, niobium-aluminum, titanium-silicon, molybdenum-silicon-boron and others are used extensively for high temperature applications due to their desirable mechanical properties. However, their environmental durability in oxidizing or harsh environments is limited and various surface modification techniques, including protective coatings are employed to extend their lifetimes and/or use temperatures. Due to presence of surface pits, scratches, pores, or other abnormal surface features (more commonly known as pitting or crevice corrosion), accelerated oxidation or corrosion is initiated in these areas which eventually degrades the entire surface. If the surfaces are prepared adequately, advanced alloys, that contain aluminum for example, will form a uniform protective alumina scale which limits further oxidation. However, if the thermally grown scale is not uniform or contain other oxides, besides that of aluminum, the protection is compromised and the alloys are subject to rapid degradation at elevated temperatures. In addition, surface grain boundary junctions are compositionally different compared to the bulk composition which may also cause the oxide scale in those regions to be different and perhaps less protective. Therefore, there is a need for a suitable surface modification method that will allow for the slow and steady formation of predominantly alumina-rich (more preferably pure alumina scale) scale for aluminum-containing alloys.
Similar arguments are valid for chromium-based steels and other chromium-based alloys which are used in applications for boilers, heat exchangers, recuperators, interconnect for solid oxide fuel cells, automotive catalytic converters, and others as apparent to those skilled in the art. In these applications, it is desired to form a protective chromia scale which requires a minimum level of chromium content in the alloy. Higher chromium content makes the alloy more expensive and also results in compromise of other important mechanical, thermal, and electrical properties of alloys. Thus, there is a need for a protective coating for chromium-based alloys and steels which will allow for the formation of a dense and uniform protective scale of chromium oxide, especially if it can be implemented for low chromium-containing alloys.
Metal or alloy honeycomb structures are used in many applications such as catalytic converters, radiators and heat exchangers, and exterior bodies of space vehicles for thermal protection. U.S. Pat. Nos. 5,411,711 and 5,146,743, among others, discuss the metal foil catalytic converters for automotive systems. Currently, most catalytic converters used in automotive exhaust systems in the U.S. use a ceramic honeycomb substrate loaded with a precious metal catalyst. The ceramic honeycomb is used because it can tolerate the hot exhaust environment without degradation. Alloy foil honeycombs offer advantages over ceramic honeycombs in weight and electrical conductivity. Most auto pollution occurs when the engine is cold, generally after the engine is started. At low temperatures the catalysts are not effective at reducing nitrogen and oxidizing residual hydrocarbons. To alleviate this problem and achieve overall reduced emissions, alloy foil catalytic converters can be resistively heated to ensure that the catalysts are kept at a temperature that allows them to function optimally. However, these thin foils are prone to oxidation and corrosion in the exhaust stream. Foils are particularly sensitive to oxidation because the original alloy is so thin, that the buildup of a thick oxide scale results in dimensional changes and changes in mechanical properties. For this reason, expensive oxidation resistant alloys are required. A thin oxidation resistant coating that will not substantially increase the thickness of the foil will be useful to reduce oxidation and corrosion, allowing the use of less expensive alloys, while still allowing the use of resistive heating to reduce emissions. Another such application is the potential use of alloy foils is for thermal protection systems for next-generation reusable launch vehicles for space travel. Present inventive material can be used as the oxidation protection coatings for these applications.
Currently, there are many ways to combat corrosion of aluminum and ferrous alloys. They include painting, electroplating, composite coverings, use of more corrosion resistant alloys, anodizing and chromating the surfaces of metal. Many of these processes are not environmentally friendly, cannot be maintained or repaired in the field, are expensive, require significant preparation of the substrate, and none offer the required long-term, low maintenance protection. Past coating efforts have primarily used relatively thick coatings (1-20 mils thick) to combat salt corrosion. Anodizing of aluminum and chromate conversion coating of aluminum and ferrous alloys are the most effective technologies, but both are environmentally unfriendly and require the use of toxic chemicals. Corrosion often occurs in areas of surface defects of the alloy substrate. Pits and inhomogeneities in the alloy composition cause accelerated corrosion. High strength aluminum alloys in particular are subject to pitting corrosion because of the influence of Cu-containing intermetallic particles. The inhomogeneous distribution of Cu in the alloy microstructure has been shown to be a major cause for low resistance to pitting or stress corrosion cracking. Heterogeneous microstructures are intentionally developed in commercial aluminum alloys to optimize mechanical properties. Unfortunately, such microstructures make aluminum alloys susceptible to localized corrosion during service and complicate aqueous surface finishing processes. The standard coating system uses a chromate conversion layer covered by organic paints. Short term corrosion protection of metals and alloys from corrosion due to moisture and other environmental factors is currently achieved using organic layers. Time-consuming and arduous, these organic coatings need to be removed before the processing of metals and alloys like heating or melting or for painting and other surface modifications.
Many metals, alloys and ceramics used in various applications require a smooth surface finish which is often accomplished by mechanical or chemical mechanical polishing means. In addition to passivation of coated surfaces, it is also desired to protect them from any environmental attack during processing or surface modification or during service. Typically, anodization of the surface with the formation of an alumina or chromia film is done to passivate the surfaces. However, the aforementioned procedures are expensive, labor intensive, and are environmentally unsafe releasing toxic substances and generating toxic waste.
Physical vapor deposition (PVD) grown amorphous silicon nitride film on metallic substrates are used for growth of single crystal magnesium oxide films using ion-beam assisted deposition (IBAD) whereby the growth is induced by e-beam evaporation, sputtering or other PVD method with another ion-beam to induce crystallographic alignment. Using this technique, biaxial texture of magnesium oxide is attained over thicknesses within 100 Angstroms as opposed to direct IBAD growth of yttria stabilized zirconia (YSZ) on highly polished polycrystalline metal or alloy substrates (hereafter referred to as metal substrates) which required growing much thicker films (over 1000 Angstroms) to attain similar quality biaxial texture. The IBAD magnesium oxide films served as good templates for further heteroepitaxial growth of functional oxide films such as ferroelectrics, superconductors, piezoelectric films, or other electronic films of the like. Thus, the IBAD MgO approach served as a much faster and economical way of producing biaxially textured or single crystal films on polycrystalline metal substrates with amorphous interlayers (also known as nucleation or adhesion layers).
It has been recently demonstrated that yttria served as a much better amorphous template layer (grown by PVD) than silicon nitride on highly polished metal/alloy substrates. Specifically, the yttria/IBAD MgO approach was used to demonstrate the architecture for growth of high quality High Temperature Superconductor (HTS) films suitable as HTS coated conductors. Specific disadvantages of this approach include: an expensive (vacuum deposition process) low deposition rate process is required for yttria amorphous layer formation, the use of thin yttria layer is not an adequate diffusion barrier against diffusion of oxygen and other metals to diffuse into the superconducting layer; thus, a separate diffusion barrier layer is still required (currently strontium ruthenate is being used as diffusion barrier), and prior to deposition of yttria, the substrate roughness needs to be tailored below 40 angstroms (preferably below 10 Angstroms) through mechanical or electrical polishing methods. Thus there is a need for an alternative material and associated thin film process (preferably non-vacuum, low-cost, and high deposition rate) to replace yttria and silicon nitride or other layers which is multifunctional and performs better and can be deposited at lower costs using a simple deposition process.
Low friction surfaces are required for many applications, including bearings, bearing races, and gears. Low friction surfaces can be imparted by depositing a low-friction material as a coating or reducing the overall surface roughness of the substrate. Although surface finish of metallic and ceramic parts can be improved through mechanical polishing, pits and defects contained on the surface cannot be effectively removed through any of the standard polishing techniques. Deposition of extremely thin amorphous films that exhibit low surface energy and provide hermetic coverage with adequate thermal and microstructural stability can be beneficial in maintaining a low friction surface whereby the defects on the metal surfaces are effectively sealed.
Biofouling of ship hulls is caused by microorganisms such as slime, algae and bacteria, and macroorganisms such as barnacles, mussels, clams and oysters which adhere to the hull of the ship. Fouling increases drag on the hull, decreasing ship speed and often significantly reducing fuel economy. One of the promising emerging technologies is the nontoxic “foul-release” coating. These coatings are based on the hypothesis that in surfaces with the weakest attraction for bio-organisms, fouling will be slow and likely to require the least amount of effort to release from the surface. Fouling organisms adhere to the surfaces by secreting proteinaceous adhesives. Materials with low surface energy will offer low adhesion strength, resulting in poor attachment and easy to remove fouling. The feasibility of this approach has been established by researchers using fluorinated polymers, epoxy based and silicone-based coatings. These coatings did foul, but fouling bio-mass can be easily removed by fast-flowing water. However, these polymer-based coatings have limited heat and UV light resistance. Therefore, an inorganic coating with smooth and low friction surface properties are highly desirable.
Microarrays are arrays of biomolecules such as oligonucleotides that are spatially arranged and stably attached to a surface of a solid support. Microarray technology is used for parallel analysis of genes in a large scale, and has emerged as the universal genetic analytical tool for use in a wide range of biomedical applications. Commercial production of DNA chips has been implemented by many companies while, in parallel, medical researchers report exciting advances across many disciplines within the field of medicine. These developments in microarray technology offer tremendous promise to solving long-standing problems in public health worldwide and also provide new avenues to combat the more recent threats of bioterrorism.
The starting point or the basic building block for producing biomolecular microarrays is a suitable solid template surface (solid support material) upon which biological molecules can be anchored or immobilized. Several patents have been issued on functionalizing silicate glass and other surfaces. Numerous other surface coatings have also been disclosed. Patents are also awarded for novel solid supports, e.g. aluminosilicate, for immobilizing nucleic acids. Characteristics of DNA microarrays are determined by the surface properties such as chemical homogeneity, interaction between surface and bio-molecules, surface roughness, density of surface functionality, spacing between surface functional moieties, amenability to DNA hybridization, and so forth. While the current methods employ the use of soda-lime glass substrates, they are prone to degradation over the long term and the surface chemistry is not tailored to allow for suitable organic attachments. An organic linker is used to attach the DNA or other biomolecule to the surface of the substrate. Polylysine is a coating material currently recommended and one of several used for glass slide preparation, as known in the art. However, polylysine-coated glass slides suffer from poor stability, extended curing cycles, and poor reliability such that new surface methodologies are critically needed to support the rapidly growing field of microarray technology. For example, polylysine-coated slides need to be stored for 14 days after coating for curing purposes and should be used within four months due to degradation from oxidation. Typically, in a batch of polylysine-coated slides, several are rejected because of non-uniformity or opacity. In addition, the hybridized microarrays cannot be stored over long time periods. Stability of polylysine coating under UV light is also a concern.
Many alternative coatings to replace polylysine are being investigated including aminosilanes, epoxy derivatives, aldehydes, and others. While aminosilanes or their derivatives offer superior stability, their low binding capacity has been a problem. Many of these limitations stem from the lack of desirable inorganic surface chemistry for bonding with organics. Organic groups functionalized on soda-lime glass surfaces are not stable under even slightly harsh conditions or chemical treatments and will degrade over time. Organic molecules interact only weakly with soda-lime-silica surfaces. Under humid or other conditions, sodium ions diffuse to the surface of the glass and interact with organic molecules resulting in degradation. Borosilicate or aluminosilicate glasses have also been proposed, but they do not offer the ideal surface chemistry for organic absorption.
Disinfecting and antimicrobial chemicals are commonly employed to eradicate microbial growth and improve hygiene. The adhesion of micro-organisms to surfaces is influenced by the bio-adhesive characteristics of the fouling organism and surface properties, such as its chemical composition and physical characteristics of the surfaces like surface roughness. Fungi, such as molds, yeasts and algae are visible in mass, but it can be advantageous to eliminate them earlier, when contamination and the consequential substrate deterioration has not yet become obvious. Highly active cleaning chemicals may be toxic and aggressive and, after repeated applications, degrade the surface and inactivate bioactive systems. Another major problem is the evolution of microbial strains which are resistant to disinfectants and antimicrobial agents that are being used now. The issue of hygiene is especially critical to contact surfaces present in food processing, supply and catering chains, health and medical establishments, animal husbandry, water and sewage operations as well as in heating, ventilation and air conditioning systems.
The performance factors of antimicrobial coatings include durability, retention of activity, and minimal degradation of surface characteristics and appearance. The coatings must also show resistance to heat, chemicals, solvents, staining, scratching, and moist environments. They should preferably be non-toxic, odorless, smooth, non-porous, easy or self clean, crack-free, avoid discoloration, have good color retention and be UV resistant. Several potential novel techniques are being developed to overcome these problems. These include albumin affinity surfaces, surface modification with blue dextran, silver ion incorporation in a porous matrix, photocatalytic titanium dioxide, silicone quartemary ammonium compounds and sacrificial coatings that are alkali soluble or strippable and recyclable films. A multi-layer film, fluoro/silicon containing resins, a dry paint film with additive coating or additives incorporated, the incorporation of cleaning agent activators, the design of surface and cleansing system in tandem, tuned ultraviolet, ultrasound and ozone could also be of value.
Among these antimicrobial techniques, there is a renewed interest in silver ion incorporation into coatings and substrates by researchers and companies. Several patents and publications have recently appeared on the use silver ion incorporated substrates like zeolites, polymers, ceramic sheets and polyelectrolyte films. Silver compounds have been exploited for their medicinal properties for centuries. It is an effective agent with low toxicity. Although silver salts are effective antimicrobial agents, their use likely results in unwanted adsorption of silver ions in epidermis cells and sweat glands. To reduce the likelihood of silver-ion adsorption into tissue, silver ions need to be incorporated into stable substrates.
The hydrophobic effect plays an important role in the defense against pathogens. In addition to the unfavorable surface energy on the hydrophobic surfaces, microorganisms are also deprived of the water necessary for germination and growth. Very few microorganisms are known to survive in the absence of water. Hence, hydrophobic property imparted on inventive material coated surfaces may be regarded as the additional defense against microbes. The combined effect of both bactericidal and hydrophobic properties of inventive material coating will act as two lines of defense against harmful microorganisms. A hydrophobic layer will prevent or reduce the adhesion of microbials and help in easy cleaning. In case of damage occurring to this hydrophobic layer during service the antimicrobial agent loaded second layer will act as second line of defense against microbes. Fiberglass insulation is used extensively in building construction. Fiberglass is an effective insulation, but is susceptible to moisture and can become a point for bacteria and mold to grow. Mold and bacteria growth in building materials causes indoor air pollution and can cause sickness in the inhabitants of the building. A water-repellent coating is desired to maintain dry conditions of the fiberglass insulation. If the fiberglass is dry, then biological growth can be prevented. Therefore, the combination of both hydrophobic and antibacterial property in one embodiment will greatly help in situation like this and others.